The invention relates to an apparatus for detecting the presence of a flame and to a method for detecting the presence of a flame.
Apparatuses for detecting the presence of a flame are used as flame monitors when monitoring combustion plants and are used as flame detectors in the field of fire prevention.
The aim of any combustion plant operator is to improve the overall efficiency of his furnace, to reduce pollutant emission and to safely monitor the combustion process with safety-related progress and optimum availability.
Radiation detectors are provided for safe monitoring, which detectors convert radiation into a measurable electrical variable according to a fixed relationship. If a definable threshold value for the measured variable is undershot, a “flame off” signal can be generated, after which the fuel supply can be switched off for safety reasons.
In the case of the radiation detectors, a distinction is made between photoelectric detectors and thermal detectors which have different radiation sensitivities and—according to the set task—are used with respect to the parameters to be recorded.
The photoelectric detectors respond to the energy quanta of radiation and their spectral sensitivity depends on the wavelength.
For reasons of price in particular, UV tubes are still used as flame sensors or flame monitors for flame monitoring, but the problem of so-called “flashovers” being able to occur in these components exists in principle. A glow discharge which cannot be distinguished from a normal flame signal by the electronics connected to the UV tube can occur without external UV irradiation.
A plurality of solutions relating to how flashovers can be detected and processed in a safety-oriented manner are known.
For example, DE 1 256 828 A discloses the practice of only periodically applying the UV radiation coming from a flame to the photocell by only periodically exposing a UV-sensitive element to the radiation by means of a rotating perforated disc. A flashover in the UV-sensitive element can be detected with a monitoring circuit, which has capacitors and transistors, in conjunction with the rotating perforated disc.
In principle, in order to detect a flashover, the incident radiation is thus periodically interrupted by a shutter mechanism. If further discharges occur in the tube in this dark phase, this is detected by the electronics connected to the UV tube, that is to say a flame relay is switched off.
A mechanical shutter mechanism which periodically interrupts the incident radiation has a limited service life on account of wear. If the times between two successive closing operations of the mechanical shutter are extended, the circuitry outlay needed to determine flashovers between the two closing operations of the shutter becomes lower; the detector element should be new and safe.
If the time between two successive closing operations of the shutter mechanism is reduced, safety is indeed increased with respect to the detection of a flashover, but the mechanical wear on the shutter mechanism also increases. In addition, deviations from the alignment of the shutter and contamination caused by abrasion, for example, may result in failure of the flame monitoring.
DE 1 293 837 A discloses a device for monitoring a pulse generator, which has a UV tube, for faults of the UV tube, in which a threshold value circuit at the output of the pulse generator is designed such that it responds only to those pulses which occur when the UV tube operates in a faultless manner. In this case, particular signal forms and values which lead to faulty detection of flashovers or extraneous radiation can be assumed.
DE 1 955 338 B describes that it is known practice to use two UV photocells which monitor the same flame and have relay circuits consisting of at least two relays connected downstream of them. The relay circuits only have a switching state which allows fuel to be supplied when the sum of the signals—a voltage drop across a series resistor—from the UV photocells exceeds a particular value and the difference between the two voltage drops undershoots a particular value. Said document describes that detection of a flashover is not important as long as the second UV photocell operates in a faultless manner. This is disadvantageous for burners which have been operating without interruption for half a year or more, with the result that it is not possible to exclude the situation in which both UV photocells also undergo a flashover during this time. DE 1 955 338 B therefore follows the route of configuring a UV flame monitor with a single UV photocell and a downstream channel without using mechanical elements. The flashover in a UV photocell is detected by virtue of the fact that a constant gas discharge at the series resistor produces a DC voltage which is used as a signal for the fault state of the UV photocell. A different radiation is required for this purpose.
DE 26 29 321 A1 discloses a device for monitoring flames in burners in furnaces, in which an electronically controllable liquid crystal shutter is fitted upstream of the optical flame sensor, through which shutter only the optical signal from the flame reaches the flame sensor and which shutter is periodically controlled from the transparent state into the darkened state. The modulated light signal from the flame sensor is evaluated via a frequency filter for flame monitoring. DE 26 29 321 A1 thus follows the route of using a liquid crystal shutter, which does not require any (precision) mechanical components, to shade the flame sensor. In practice, this use results in high attenuation of the UV radiation and in insensitivity which no longer suffices for the desired measurement purposes.
It becomes clear from the above that a plurality of solutions have been proposed for designing a safe apparatus for detecting the presence of a flame and a method for detecting the presence of a flame, which reliably detects flashovers and a malfunction. However, the proposed solutions all have a disadvantage which results in a high degree of wear on and/or a complicated design of mechanical components or electronic circuits, in which case compromises are accepted.